[C++] Add method to append batches of null-terminated strings to StringBuilder

[asfimport]

We should add a method StringBuilder::AppendCStrings(const char** values, const uint8_t* valid_bytes = NULLPTR) to the StringBuilder class to have fast inserts for these strings. See https://github.com/apache/arrow/pull/1845/files for a use case.

Reporter: Uwe Korn / @xhochy
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #1852

_{Note: This issue was originally created as ARROW-2411. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 1852
#1852